1. Field of the Invention
The invention relates in general to transport refrigeration systems, which hold a set point temperature by way of heating and cooling cycles, and more specifically to such systems which utilize hot compressor discharge gas for heating.
2. Description of the Prior Art
In the transportation of perishable products, it is recognized that it is necessary to provide refrigeration for the cargo space. It is also well known that it is necessary to periodically provide heat to remove the accumulation of frost or ice from the refrigeration system evaporator. Also, when transporting perishable products through areas having a cold climate, it is necessary to provide heat to the cargo space to prevent excessive cooling or freezing of the perishable products.
A well known method of providing heat for defrost and heating cycles is to divert hot compressor discharge gas from the normal refrigeration circuit directly to the evaporator to achieve the desired heating. It has been recognized that when such a switch from a cooling cycle to a heating cycle is made that a substantial amount of the refrigerant in the system is trapped in active parts of the system and accordingly not available for providing heat.
U.S. Pat. No. 3,219,102 "Method and Apparatus For Deriving Heat From Refrigerant Evaporator" teaches a system for delivering a hot compressed gas from the compressor to the receiver, to pressurize the receiver and thus force liquid refrigerant from the receiver into the remainder of the refrigeration circuit.
U.S. Pat. Nos. 4,748,818 and 4,912,933, both entitled "Transport Refrigeration System Having Means For Enhancing the Capacity Of A Heating Cycle", disclose a system that includes a refrigerant line that establishes fluid communication between the receiver and the suction accumulator in such a system. The '818 patent teaches that the refrigeration system simultaneously 1. Shift to the Heating Mode; and 2. Establish a fluid flow communication between the accumulator and the receiver. The '933 patent makes the interconnection between the accumulator and the receiver just prior to each heating cycle, while maintaining a cycle control valve in the cooling position for a pre-determined time delay. This forces any liquid refrigerant trapped in the condenser to flow into the receiver. A second mode of operation of the '933 patent calls for the establishment of the fluid communication between the accumulator and the receiver prior to the initiation of the heating cycle and further continues this fluid communication during the duration of the heating cycle.
Each of the '818 patent and the '933 patent may effectively serve to draw additional refrigerant into the heating circuit upon the initiation of a heating cycle. Once this initial withdrawal of refrigerant occurs, however, the capability of systems according to the '818 and '933 patents to draw additional refrigerant into the system is limited by the temperature/pressure of the condenser which, during heating is at ambient temperature. Such a limitation may create refrigerant inventory problems during periods of extended heating operation, particularly at low ambient temperatures.
It has been recognized that, during periods of extended continuous heating, minor valve leaks in transport refrigeration systems will result in refrigerant migrating back into the condenser and the refrigerant lines which are not active during the heating cycle. It will be appreciated that this tendency can be further aggravated during low ambient temperature conditions when the condenser coil represents the coldest point of the system.